The present disclosure relates to a preferred device configuration used in a wavelength sweeping type optical coherence tomography.
An optical coherence tomography (hereinbelow referred to as “OCT”) is for measuring a tomographic image of a living subject by using interference of light, and is commonly used especially in ophthalmology as a method of obtaining two-dimensional or three-dimensional tomographic images of a cornea and a retina. In recent years, a Fourier domain method has been employed. The Fourier domain method includes a spectral domain OCT (hereinbelow referred to as SD-OCT) where a tomographic image is obtained by detecting spectral information using a spectroscope, and a swept source OCT (hereinbelow referred to as SS-OCT) where a tomographic image is obtained by detecting a spectral interference signal using a wavelength sweeping light source. The SS-OCT is also termed optical frequency domain imaging (OFDI), and is a technique the same as the OFDI (for example, see Japanese Patent Application Publication No. 2007-510143).
In the Fourier domain method, light is introduced to an interferometer from a light source of which wavelength has a broad band, and the light is outputted to a subject such as an eyeball. Scattered light is reflected from the subject, and the scattered light is detected by the interferometer. A raw signal detected (interference signal) is an interference spectrum, and a distribution of scattered light intensity scattered in a depth direction of the subject is obtained by performing Fourier analysis on the interference signal. Further, a two-dimensional distribution of scattered light intensity or a three-dimensional distribution of scattered light intensity can be obtained by scanning a beam irradiated to the subject in a lateral direction, and this distribution of scattered light intensity corresponds to a tomographic image of the subject. This is termed OCT tomographic image or the like.
In the SD-OCT, the interference spectrum is obtained by using light of which wavelength has a broad band as a light source and an array sensor via an interferometer for detection. In the SS-OCT, since wavelength sweeping light of which wavelength is swept with high speed is used, the detection of the interference spectrum is obtained as a time-domain function.
However, in both of the methods, due to the nature of Fourier transform, a mirrored image of the OCT tomographic image appears symmetrically relative to a zero frequency corresponding to a zero point and relative to a Nyquist frequency being a depth where a detection range terminates. The mirrored image relative to the latter frequency can be easily eliminated by a filter, however, the mirrored image relative to the zero frequency is difficult to be eliminated and is disincentive as an artifact in the measurement. Hereinbelow, only the mirrored image relative to the zero frequency will be considered.
In both of the OCT methods of the Fourier domain method, the broader the wavelength band to be used is, the higher resolution capability becomes.
However, in the SD-OCT, a low-coherent light source is used in principle, due to an increase in a wavelength band width. Therefore, with the high resolution capability, sensitivity for the interference signal largely reduces as a depth of the image increases. To solve this problem, wavelength resolution capability of the detector needs to be enhanced, however, this is difficult in terms of a configuration of the detector. On the other hand, in the SS-OCT, since the wavelength is swept timewise and coherence of the light source is determined by a substantially instantaneous line width, the directly opposing influence of the sensitivity reduction relative to the high resolution and the depth is not big.
As the SS-OCT, an attempt using an SSG-DBR (Super Structure Grating Distributed Bragg Reflector) laser of a communication band (for example, see Japanese Patent Application Publication No. 2005-156540), and a method using a multimode wavelength sweeping light source with an external cavity including a polygon scanner (for example, see Japanese Patent Application Publication No. 2007-510143 and Japanese Patent Application Publication No. 2007-526620) have been suggested. Especially, the latter has been used as a practical ophthalmic OCT. Further, in recent years, a technique which is sped up by using a MEMS (microelectromechanical system) has been suggested, and not only the multimode wavelength sweeping light source but also a single-mode wavelength sweeping light source such as a DBR laser and a VCSEL (vertical cavity surface emitting laser) delivers practical performance (for example, see U.S. Pat. No. 7,468,997, Japanese Patent Application Publication No. 2006-337239, Japanese Patent Application Publication No. 2007-278868, Japanese Patent Application Publication No. 2014-522105, Japanese Patent Application Publication No. 2015-102537, and Japanese Patent Application Publication No. 2015-523578).
A characteristic substantially largely different between the multimode wavelength sweeping light source and the single-mode wavelength sweeping light source lies in the instantaneous line width. The latter usually oscillates light with a narrower line width. The line width is strongly associated with the coherence. Specifically, using a coherence length for evaluation of coherence, a coherence length of the multimode wavelength sweeping light source is approximately 20 mm at longest, whereas a coherence length of the single-mode wavelength sweeping light source can be equal to or greater than 100 mm By making a condition of optical interference signal measurement appropriate, the tomographic image can be obtained in a depth range corresponding to the coherence length. In reality, since reflection and the scattered light of the subject are detected, the depth range in which the tomographic image can be obtained with an effective sensitivity is approximately a half of the coherence length.
For example, since even the multimode wavelength sweeping light source has a sufficient coherence in an anterior eye and a posterior eye, it has been applied as an ophthalmologic instrument. Further, the single-mode wavelength sweeping light source expands a possibility of an OCT or an eye axial length measurement device which can target all kinds of eyes (16 mm to 40 mm).